wcn36xx: Implement get_snr()

The wcn36xx BD phy descriptor returns both Received Signal Strength
Information (RSSI) and Signal To Noise Ratio (SNR) with each delivered BD.

The macro to extract this data is a simple-one liner, easily imported from
prima driver. This data will be useful to us when implementing
mac80211-ops->get_survey().

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220115001646.3981501-2-bryan.odonoghue@linaro.org